It is well known to improve the oxidation and/or corrosion resistance of various nickel, cobalt and/or iron base superalloy gas turbine engine components by forming a protective aluminide diffusion coating thereon. One coating process used to form aluminide coatings on superalloy substrates involves a pack process (pack cementation) wherein the substrate to be coated is placed in a particulate bed of coating material (e.g. aluminum particulates, halide activator, and inert diluent) and heated in a retort to an elevated coating temperature to generate an aluminum halide coating gas that reacts with one or more substrate elements (e.g. Ni for a nickel base superalloy substrate) to form the aluminide coating (e.g. nickel aluminide) on the substrate. U.S. Pat. Nos. 3,544,348, 3,961,098, 4,070,507, and 4,132,816 disclose pack coating processes for forming aluminide coatings.
Another coating process used to form aluminide coatings on superalloy substrates involves a chemical vapor deposition process (CVD process) wherein the substrate to be coated is placed in a reactor, heated to elevated coating temperature, and a coating gas comprising aluminum halide (e.g. Al trichloride or subchlorides) and a carrier gas, such as hydrogen, is introduced to the reactor to deposit aluminum on the substrate for reaction therewith to form the aluminide coating thereon. The Gauje U.S. Pat. No. 3,486,927 represents an early effort directed to a CVD coating process.
The CVD process can be used to form aluminide coatings including additional alloying elements in the coating. For example, U.S. Pat. No. 2,772,985 discloses formation of binary Al—B, Al—Si, Al—Ti, and Al—Zr coatings on Mo substrates. The patented process first vapor phase deposits Al on the Mo substrate and then reacts the deposited Al with the alloying element (B, Si, Ti or Zr) deposited from a second coating gas. The aluminum halide coating gas and alloying element halide coating gas are generated in separate parallel generators to provide separate coating gas streams. The process does not involve codeposition of the coating elemental constituents on the substrate.
U.S. Pat. No. 4,687,684 relates to CVD formation of oxidation resistant Si and Cr-enriched aluminide diffusion coatings on a superalloy substrate. The patent provides for sequential introduction of the aluminum halide coating gas and the silicon or chromium halide coating gas to the coating reactor. The patent does not involve codeposition of the coating elemental constituents on the substrate.
U.S. Pat. No. 5,015,502 discloses a CVD coating process for forming aluminide coatings containing a gettering element (e.g. Y) by chlorination of a source of aluminum containing Y and one or more of Si, Cr, Co, etc. A preferred source is described as an Al—Y—Si alloy.
Aluminum and silicon have been intermittently codeposited on a heated superalloy substrate by CVD such that a silicon-modified aluminide coating is formed. In particular, low silicon contents of trace to 8 weight % are desirable to improve the oxidation/corrosion resistance of the aluminide coating. Since typical silicon precursors, such as silicon tetrachloride and dichlorosilane, are much less stable than aluminum trichloride, a short pulse (e.g. 7.5 minutes) of the silicon halide or silane coating gas is provided intermittently (e.g. once every two hours) during the continuous CVD aluminum deposition to obtain the desired silicon concentration in the coating. Since no intermetallic compounds form between aluminum and silicon and the coating is applied at high temperature, silicon can be deposited on the substrate from pulses of silicon tetrachloride or dichlorosilane and then it diffuses during the aluminizing cycle to give a homogenous silicon modified aluminide coating.
If Al and a reactive element (such as Hf, Zr and Y) which form stable intermetallic compounds with Si are continuously codeposited while Si is intermittently codeposited by the aforementioned CVD pulsing technique, then the resulting modified aluminide coating will be heterogenous in that the pulses of silicon coating gas produce layers in the coating that contain a large volume fraction of silicon rich intermetallic compounds.
Such a coating structure wherein the majority of the reactive element (gettering/active element) exist as intermetallic compound layers does not exhibit the superior oxidation resistance of a modified aluminide coating wherein the reactive element is uniformly dispersed throughout the aluminide coating.
Thus, there is a need for the continuous deposition of aluminum and a reactive element, such as Hf, Zr, and/or Y, with or without Si to obtain a more homogenous coating for oxidation resistance.
It is an object of the invention to provide a CVD method and apparatus for codepositing multiple elements on a substrate in a manner to overcome disadvantages discussed above.
It is another object of the invention to provide a CVD coating method and apparatus for forming aluminide coatings wherein aluminum and one or more reactive elements for surface active superalloy substrate impurities (e.g., S, P, etc.) are codeposited on a substrate.
It is still another object of the invention to provide a CVD coating method and apparatus for forming an aluminide coating including silicon and one or more reactive elements for surface active superalloy substrate impurities wherein aluminum, silicon and the reactive element(s) (e.g. Hf, Zr, Y) are codeposited on the substrate.
It is still a further object of the invention to provide a coated superalloy article having an aluminide coating thereon including one or more reactive elements for surface active superalloy substrate impurities uniformly distributed in a region of the coating or throughout the coating for improved oxidation resistance.